1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator mounted in communications equipment such as a cellular phone, and the like.
2. Description of the Related Art
A temperature-compensated crystal oscillator mounted in communications equipment comprises a crystal oscillation circuit incorporating an AT-cut crystal resonator (unit) in a frequency band of 10 MHz as the oscillation source thereof, and a temperature compensation circuit using a frequency adjustment circuit for adjusting an oscillation frequency of the crystal oscillation circuit so as to stabilize the oscillation frequency by canceling out the cubic curve temperature characteristic of the AT-cut crystal resonator.
Among such temperature-compensated crystal oscillators, a direct-compensation type analog temperature-compensated crystal oscillator made up of discrete components has been in the mainstream up to now.
However, in light of a recent trend of increasingly adopting the CDMA (code division multiple access) system aiming at international sharing of a common communications system, an indirect type temperature-compensated crystal oscillator capable of expanding the range of temperature compensation has attracted much attention.
With the indirect type temperature-compensated crystal oscillator, a temperature compensation signal is generated by use of some type of circuit, and temperature compensation for the AT-cut crystal resonator is achieved by controlling a variable capacitance circuit, and the like, with the signal.
As a circuit for generating the temperature compensation signal, a resistor circuit network, made up of discrete components, was used in the past, but a semiconductor IC has recently come into use in many cases.
Since the temperature compensation signal is generated mostly as a voltage signal, a variable capacitance circuit of a voltage control type is often used in the frequency adjustment circuit There has been a shift in the. driving voltage of the temperature-compensated crystal oscillator mounted in the cellular phone from 5 V to 3 V, and further reduction in the voltage is called for.
Consequently, a voltage range of a signal impressed on the variable capacitance circuit has gradually become less.
Accordingly, with the indirect type temperature-compensated crystal oscillator, a variable capacitance circuit having a large capacitance variation ratio in a narrow voltage range is in demand in order to expand the range for temperature compensation.
Also, a system has since been proposed whereby the temperature compensation signal and an external frequency control signal are synthesized into a composite signal for controlling the variable capacitance circuit so as to eliminate mutual interference between temperature compensation and external frequency control for controlling the oscillation frequency by an external signal.
Such a proposal, however, is based on the premise that the variable capacitance circuit has a large capacitance variation ratio such that the oscillation frequency of the crystal oscillation circuit can be varied steeply with a small change in voltage.
Hence, in the indirect type temperature-compensated crystal oscillator, the variable capacitance circuit, in particular, is an important building block thereof.
The variable capacitance circuit comprises at least one variable capacitance element An example of a conventional variable capacitance circuit is shown in FIG. 16.
In the conventional variable capacitance circuit, a fixed capacitor 45 serving as a DC cutoff capacitor and a variable capacitance element 49 are coupled in series, and the other terminal of the variable capacitance element 49 is grounded, while the other terminal 46 of the fixed capacitor 45, as the output terminal thereof, is coupled to the crystal oscillation circuit. An input resistor 47 is connected to a node between the fixed capacitor 45 and the variable capacitance element 49. A control signal A is applied to the node via the input resistor 47.
The input resistor 47 has the function of blocking AC signals, and may be dispensed with provided that a circuit for generating the control signal has a sufficiently high output impedance. The other terminal of the variable capacitance element 49 may sometimes be coupled to a power source (Vcc) on the higher potential side.
A variable capacitance diode, MOS capacitor, and the like are typical of the variable capacitance element 49.
In either of the variable capacitance diode or MOS capacitor, advantage is taken of a phenomenon in which the width of the depletion layer of a semiconductor changes according to voltage, and some ingenious ideas have been devised for application in the manufacturing stage thereof in order to increase the capacitance variation ratio.
In the case of the variable capacitance diode, a type of pn junction diode, an attempt has been made to provide the impurity concentration profile on the side towards which the depletion layer is widened, that is, on the lightly doped side, with a gradient, or to lower the impurity concentration.
Also, in the case of the MOS capacitor, an attempt has been made to lower the impurity concentration in the semiconductor substrate thereof, or to render the thickness of the gate oxide film thinner.
However, since lowering of the impurity concentration in a semiconductor has its limitations, the minimum capacitance value of the variable capacitance cannot be reduced to a great extent, and moreover, there will arise a problem that if an attempt is made to reduce the minimum capacitance value by lowering the impurity concentration, the maximum capacitance value in the range of application voltages also becomes smaller.
Further, in the case of the MOS capacitor, there is a problem in that the maximum capacitance value in the range of application voltages cannot be increased to a great extent even if the maximum physical capacitance value is increased by rendering the thickness of the gate oxide film thinner.
After all, in spite of all the attempts tried at the stage of manufacturing the variable capacitance element up to now, the capacitance variation ratio could not be increased satisfactorily in the range of application voltages.